Um. OK. They can be a trap.
For starters, check your annual fee. That can be most of a cheap flight.
Check how much you earn per dollar in points. Then check to see how many of those points convert to FF points for flights. Often its 2:1 so you get half.
If you don't pay your card off every month, you are not getting anything as the fees and interest will be worth more than the points.
Check to see if there is a limit on how many points you can earn in a month.
Generally, if you are rich and spend up big on credit cards but can pay them off every month, they are worth the effort. Otherwise, just keep an eye out for flight specials. I am told by those in the know that they are better for upgrading than actually buying flights. So you buy a cheap seat then use your points to upgrade to something more comfortable. Just check that the airline will let you do it though as some sectors they don't allow that.